•
20.5
irradiance deviates from the cell efficiency at the 1000 W/m
2
irradiance at STC
(see Section 20.3.3).
The ohmic losses on an annual basis are always lower than the ohmic losses at
STC, because of the irradiance distribution over the various irradiance classes.
The product of the performance ratio and the in-plane irradiance gives the
specific annual energy yield.
Designing stand alone PV systems
In this final section, we take a closer look at the design of stand alone PV systems (also
called off-grid systems). Choosing a good design is more critical for stand alone systems
than for grid-connected systems. The reason for this is that stand alone systems cannot fall
back on the electricity grid, which increases the requirements on their reliability.
A major component of stand alone systems is the storage component, which can store
energy in times when the PV modules generate more electricity than required and it can
deliver energy to electric appliances when the electricity generated by the PV modules is
not sufficient. A major design parameter for stand alone systems is the required number of
autonomous days, i.e. the number of days a fully charged storage component must be able
to deliver energy to the system until discharged.
The sizing of the PV array and the storage component, usually a battery bank, are
interrelated. Here, two parameters arise. First, E fail , which is the energy required by the
electric load that cannot be delivered by the PV system, for example if the batteries are
emptied after several cloudy days. Secondly, E dump , which is the energy produced by the
PV array that is neither used for driving a load nor is stored in the batteries, for example, if
the batteries are already full after a number of sunny days. We can define the loss of load
probability (LLP),
where P L is the load power. Note that the lower the LLP, the more stable and reliable the
PV system. Table 20.4 shows recommended LLPs for different applications.
Table 20.4: Recommended Loss of Load Probability (LLP) for some Applications [176].
Application
Recommended LLP
Domestic illumination
10 −2
Appliances
10 −1
20.5
irradiance deviates from the cell efficiency at the 1000 W/m
2
irradiance at STC
(see Section 20.3.3).
The ohmic losses on an annual basis are always lower than the ohmic losses at
STC, because of the irradiance distribution over the various irradiance classes.
The product of the performance ratio and the in-plane irradiance gives the
specific annual energy yield.
Designing stand alone PV systems
In this final section, we take a closer look at the design of stand alone PV systems (also
called off-grid systems). Choosing a good design is more critical for stand alone systems
than for grid-connected systems. The reason for this is that stand alone systems cannot fall
back on the electricity grid, which increases the requirements on their reliability.
A major component of stand alone systems is the storage component, which can store
energy in times when the PV modules generate more electricity than required and it can
deliver energy to electric appliances when the electricity generated by the PV modules is
not sufficient. A major design parameter for stand alone systems is the required number of
autonomous days, i.e. the number of days a fully charged storage component must be able
to deliver energy to the system until discharged.
The sizing of the PV array and the storage component, usually a battery bank, are
interrelated. Here, two parameters arise. First, E fail , which is the energy required by the
electric load that cannot be delivered by the PV system, for example if the batteries are
emptied after several cloudy days. Secondly, E dump , which is the energy produced by the
PV array that is neither used for driving a load nor is stored in the batteries, for example, if
the batteries are already full after a number of sunny days. We can define the loss of load
probability (LLP),
where P L is the load power. Note that the lower the LLP, the more stable and reliable the
PV system. Table 20.4 shows recommended LLPs for different applications.
Table 20.4: Recommended Loss of Load Probability (LLP) for some Applications [176].
Application
Recommended LLP
Domestic illumination
10 −2
Appliances
10 −1